Usually refurring to when a girl has a mustache. A Dirty Pistachio can be seen if you watch the porn film, Hairy Lesbians Gone Wild, on the pornstar Morgan Lewis. This normally means that this chick has a longer mustache than the man!

Wow that cougar has a Dirty Pistachio
OR
I had sex with that Dirty Pistashio last night
OR
I only date girls with Dirty Pistashios